Particles of matter are always moving and are never at rest. At a given temperature in any gas, liquid or solid, there are particles with different amounts of kinetic energies. In the case of liquids, a small fraction of particles at the surface, having higher kinetic energy than the bulk is able to break away from the forces of attraction of other particles and gets converted into vapour. This phenomenon of change of a liquid into vapours (gases) at any temperature below its boiling point is called evaporation.